There are two types of spindles on the market, i.e. conventional and reciprocating spindles. Conventional spindles cover a wide range of revolutions per minute (RPM) up to 300,000 RPM, and provide only rotation to the drill bit, not a feeding motion into the work piece, which is provided by a system to move the spindle. These spindles are typically relatively heavy, e.g., weighing between 4 and 7 kilograms, and require considerable energy to accelerate and decelerate in a feeding movement. The present trend towards small drill bit diameters demands good machine stability which is very difficult to obtain with high weight spindles. Reaction forces from accelerating and decelerating a heavy spindle generate vibrations on the machine which are detrimental to small-hole drilling.
Reciprocating spindles provide both rotation to the drill bit as well as a feeding motion. The reciprocating spindle typically has a very low moving mass since only the shaft and driving coil are moving, e.g., in the neighborhood of 0.45 kilograms. This is well suited for small-hole drilling because it generates very small forces during drilling; however it has drawbacks. One is that the stroke of the spindle is limited in contrast to the conventional spindle where it can be as large as required. Secondly it is difficult to run high RPMs with any practical stroke.
The majority of PCB drilling machines use spindles with air actuated spring loaded tapered collets. This solution is very complex, contains 20 to 35 parts, and is difficult to balance, expensive to manufacture and hard to maintain low run-out. In addition the taper collet configuration is affected by centrifugal force which causes it to reduce the grip on the drill bit as RPMs increase.
There are many existing centrifugal collets. Examples of collets are disclosed in U.S. Pat. No. 6,443,462B2, and U.S. Pat. No. 5,997,223.